[tor-dev] Notes from the prop224 proposal reading group
desnacked at riseup.net
Thu Mar 17 11:18:07 UTC 2016
so we had a meeting about the future of "Next Generation Hidden Services" aka prop224.
It was a good meeting.
We spent most of the time discussing the topics brought up here:
Please read the above mail to get up to speed with the topics of discussion.
So these are basically two topics:
a) Should we try to maintain backwards compatibility with old intro/rend points?
We ended up deciding that it's actually worthwhile keeping backwards
compatibility if we can help it: Our rationale was that coding the logic for
the extra cells that are required to maintain backwards compatibility is
probably less painful than coordinating a perfect bug-free incremental
deployment of prop224 in such a way that the whole network has upgraded
before we enable client support.
So our current plan is to bake backwards compat in the code, and also
introduce a consensus parameter of "Should we connect to prop224 onion
services using legacy IPs/RPs?" which we will switch off when a good
percentage of the network has upgraded to support prop224. At that point, we
will be able to rip out the backwards compat code as well. As an added
point to that, we should implement the backwards compat code in a way that
it's not spagghetied with the normal code, so that it's easy to analyze and
remove when needed. Furthermore, we should look back in prop224 and see if
we can simplify the backwards compat logic even more.
FWIW, prop224 does not have backwards compatibility for HSDirs, so we will
still have to do careful deployment there, and make sure that a good
percentage of HSDirs have upgraded to prop224 before we start using them
(maybe a new consensus parameter is needed here).
b) In prop224, why do intro points need to know the "intro point encryption key"
and also what's the point of UPDATE-KEYS-SUBCMD?
Nick told us that the main point of UPDATE-KEYS-SUBCMD is that so hidden
services can rotate their intro point encryption key periodically, so that
they can reset their replay caches.
That's a fair point. The big question here is, is this worth the complexity
that MAINT_INTRO and UPDATE-KEYS-SUBCMD add to the protocol logic?
Unfortunately we don't know the answer to this yet, because we actually
don't have stats on how our replay caches are doing currently. Here are some
things we need to look into:
i) Speak with facebook or some other big onion hoster, and try to get some
information about their replay cache. Is it ready to bust? How much space
is it occupying?
ii) The above will help us gauge replay caches in terms of normal activity,
but we also need to think of how replay caches should act in cases of a DoS.
iii) Also think about INTRODUCE1 replay attacks in general, and understand
how dangerous they can be, so that we know how robust our replay caches
should be. Is it _that_ dangerous if an attacker manages to sneak in a
replay attack every once in a while?
For example, imagine an attacker who fills up our replay cache on
purpose, so that she can sneak in one replay attack when we reset
it. If an attacker can send legit INTRODUCE2 cells to fill up our
replay cache, what does she need the replays for? What other types of
attacker are there?
After we learn more about the above, we will understand *how much we care*
about onion services being able to rotate intro pint encryption keys on the
fly using UPDATE-KEYS-SUBCMD. Here are some possible scenarios here:
- We end up deciding that INTRODUCE1 replay attacks are *dangerous* and we
need to seriously defend ourselves against them. In this case, having some
sort of UPDATE-KEYS-SUBCMD mechanism sort of makes sense, since onion
services should be able to rotate keys at will and let their clients know,
without them having to fetch a new HS descriptor.
- We end up deciding that INTRODUCE1 are a concern but not something we
should complicate our design considerably for. In this case, we might want
to consider improving the quality of our replay caches, by using more
compact designs (= more space for entries), or by using bloom filters or a
similar tech. A proposal will need to be made if so. Yawning posted the
following relevant paper: https://home.kookmin.ac.kr/~mkyoon/publications/2010TKDE.pdf
- We end up deciding it's no big deal if the attacker sneaks in a replay
attack every once in a while. In this case, we can just go with the most
minimal approach and ditch the UPDATE-KEYS-SUBCMD mechanism for now, and
have intro points be oblivious about encryption keys (which further
simplifies our cell formats, etc.). If we take this approach, and in the
future future we change our minds about the importance of replay attacks,
we can still add some sort of UPDATE-KEYS-SUBCMD mechanism using the
extensions mechanism of ESTABLISH_INTRO cells.
Furthermore on this topic, we discussed some alternate approaches for doing
replay detection that would require less heavy replay caches:
i) Maybe we can have an increasing counter in the HS descriptor, that
clients need to also include in their INTRODUCE1 cell. If an onion
service receives an INTRODUCE1 cell with a counter value different than
the one in the descriptor, it ignores it. Onion services update the
counter and publish a new descriptor when their replay cache is close to
I believe various subtle engineering details are hidden in this scheme,
along with edge cases and race conditions like "What happens if the
client fetches a descriptor with counter value N, right before the onion
service updates the descriptor with a new counter value?".
ii) When an intro point replay cache is almost full, the onion service will
make new encryption keys, tear down the old intro circuit and make a new
one, and also pass a signed "HS moved to another circuit" message to the
intro point. The intro point will forward that message to any clients
that ask for the onion service using its old keys, so that clients know
the right place to ask. Lots of engineering details hidden in this
proposal as well.
In the end we decided that Roger might have some useful feedback for us here :]
More research is required!
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